In barrier-based toll collection systems and other types of transponder supported operations, the separation of transponder and non-transponder equipped vehicles is a key system design feature. Conventionally, this process is accomplished by the use of the antenna pattern of the transponder/reader communication system. The antenna is focused on the decision area and if communications occur in this antenna pattern area, then the vehicle is declared to have a transponder and electronic processing is utilized. The problem with this approach is that the antenna pattern must be small enough to avoid communicating with vehicle transponders adjacent to the decision area and associating that communications with a non-transponder equipped vehicle in the decision area. These adjacent transponders can be behind the vehicle in the decision area, i.e., "bumper to bumper reads" or can be in an adjacent lane, i.e., "cross lane reads."
Another problem occurs when multiple vehicles, each with transponders, arrive in a sequence at the barrier. If the system communicates with the vehicles in a different order from the order in which they physically arrived, an error will be made since they will be processed in the "electronic order" not the physical order. This error, i.e., an "out of sequence read," associates the wrong vehicle with a transmission and can result in a vehicle being processed incorrectly.
Conventional systems have attempted to solve these problems in four basic ways. First, the communications area of the antenna is made as small as possible while trying to maintain reliable processing. Second, the data processed is kept to a minimum or vehicle speed is limited so that the amount of time that the vehicle is in the communication zone allows for multiple communications tries, thus improving the performance. Third, the design of the barrier and mounting of the antenna attempts to minimize and control multi-path effects so that the communication zone has a minimum number of potential fades. Fourth, the transponder is mounted in the vehicle so that all vehicles have the same antenna pattern and communication coverage. In the extreme, this means that the transponders are "trimmed in power" to achieve consistent communication coverage.
Different techniques have been employed to accomplish these error reduction functions. In one approach, a directive antenna illuminates a small area of road and limits the communication zone area by using a backscatter communication technique. This provides a path loss attenuation that increases at a rate of R.sup.4 and, thus improves coverage control. In addition, the transponder is in a specific place in the vehicle and a vehicle detector activates the communication process. This technique makes transponder location critical and the transponder antenna pattern variation from vehicle to vehicle causes many processing errors. The use of the vehicle detector to activate the process minimizes processing errors but decreases collection performance since the probability of vehicle detection is then multiplied times the communication processing probability. Specified performance values of greater than 99.995% probability of correct processing makes this approach unattractive.
In another approach, antennas buried in the road are used for communication with the transponder equipped vehicles. The transponders are then mounted on the bumper of the vehicle thus controlling the communication zone used to perform the processing. The road antennas are expensive to install and failure requires penetration of the road surface for repair, therefore shutting down of the traffic lane. In addition, the mounting of the transponder on the bumper requires a much more rugged housing and makes difficult any vehicle operator interface with the system processing results.